Extractive electrospray ionization

Analyte ions can also be efficiently generated when sample vapor or finely dispersed sample droplets transported by a carrier gas stream are admixed to the expanding electrospray plume. This technique, simple yet effective, has been introduced as extractive electrospray ionization (EESI) [37]. It utilizes two separate sprayers, one conventional ESI sprayer to provide the electrostatically charged mist and another to supply the sample vapor or mist (Fig. 13.13). While this approach is suggested for API interfaces with the heated transfer capillary design, the sample carrier stream may alternatively be passed into the desolvation gas of interfaces employing the heated curtain gas design (Fig. 13.14) [6,38]. 

Instead of a separate sample sprayer, an ultrasonic nebulizer may also deliver a sample-containing aerosol, which is transported and admixed to the electrospray mist by action of a mild stream of nitrogen. As the droplets of both origins are fused inside a small housing enclosing the spray, this approach has been termed jused-droplet electrospray ionization, and thus, led to the the somewhat confusing acronym FD-ESI [39,40]. 

3 Electrospray-Assisted Laser Desorption/lonization (ELDI) 

The technique of electrospray-assisted laser desorption/ionization (ELDI) combines two well-matured techniques of ionization for the benefit of inproved analysis of samples under ambient conditions. The development of ELDI emanates from the fact that in (MA)LDI by far more neutrals an ions are released from the sample layer (Chap. 11) [45]. Consequently, post-ionization of laser-desorbed neutrals is promising and such methods have indeed been developed (cf. Refs, in [46]). The unique feature of ELDI is to laser-irradiate the sanple in the ambient close to the ESI plume, wherein the neutrals are then ionized by ion-molecule reactions [46]. 

Example The combined use of ESI and LDI in ELDI is demonstrated for a sample of neat bovine cytochrome c on a stainless steel target (Fig. 13.16). The first spectrum (A), basically a blank spectrum, was obtained under pure LDI conditions. Spectrum B was measured with ESI on, but no laser irradiation. Finally, spectrum C was generated with both ESI and LDI active. It clearly shows the charge state distribution of the protein as typical for ESI spectra. The advantage of 

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Chen, H., Wortmann, A., Zhang, W. and Zenobi, R. (2007) Rapid in vivo fingerprinting of nonvolatile componnds in breath by extractive electrospray ionization qnadrnpole time-of-flight mass spectrometry. Angew Chemie, 119 (4), 586-589. 

Williams, J.P. Scrivens, J.H. Coupling desorption electrospray ionization and neutral desorption/extractive electrospray ionization with a travelling-wave based ion mobility mass spectrometer for the analysis of drugs. Rapid Commun. Mass Spectrom. 2008, 22, 187-196. 

Chen, H. Yang, S. Wortmann, A. Zenobi, R. Neutral desorption sampling of living objects for rapid analysis by extractive electrospray ionization mass spectrometry. Angew. Chem. Int. Ed. 2007, 46, 7591-7594. 

Several other ionization methods have been developed based on DESI, including desorption atmospheric pressure chemical ionization (DAPCI), desorption atmospheric pressure photo-ionization (DAPPI), laser ablation electrospray ionization (LAESI), and extractive electrospray ionization (EESI). Each technique uses variations of the solvent, how the charged beam is formed, and how the beam is nsed to facilitate the prodnction of analyte ions. Because these are surface methods (except EESI), they are incompatible with LC. 

Chen H, Venter A, Cooks RG. Extractive electrospray ionization for direct analysis of undiluted urine, milk and other complex mixtures without sample preparation. Chem Commun (Camb) 2006(19) 2042-2044. 

Chen H, et al. Neutral desorption sampling coupled to extractive electrospray ionization mass spectrometry for rapid differentiation of biosamples by metabolomic fingerprinting. J Mass Spectrom 2007a 42 1123-1135. 

Fused Droplet Electrospray Ionization and Extractive Electrospray Ionization... 

Chen, H., Venter, A., Cooks, R.G. (2006) Extractive Electrospray Ionization for Direct Analysis of Undiluted Urine, Milk and Other Complex Mixtures Without Sample Preparation. Chem. Commun. 2042-2468. 

Marquez, C.A., Wang, H., Fabbretti, F, Metzger, J.O. (2008) Electron-transfer-catalyzed Dimerization of fratis-Anethole Detection of the Distonic Tetramethylene Radical Cation Intermediate by Extractive Electrospray Ionization Mass Spectrometry. J. Am. Chem. Soc. 130 17208-17209. 

Zhu, L., Gamez, G., Chen, H.W, Huang, H.X., Chingin, K., Zenobi, R. (2008) Real-Time, On-line Monitoring of Organic Chemical Reactions Using Extractive Electrospray Ionization Tandem Mass Spectrometry. Rapid Commun. Mass Spectrom. 22 2993-2998. 

In more recent work, Gu et al. have reported a geometry-independent neutral desorption device [57]. The instrument uses a neutral desorber gas which is directed onto the surface to remove the adsorbed analytes. A multistage extractive electrospray ionization mass spectrometer (EESI-MS) was used to successfully obtain spectra of traces of volatile explosives (i.e., TATP) and nonvolatile explosives, (i.e., 2,4,6-trinitrotoluene [TNT], 1,3,5-trinitroperhydro-l, 3,5-triazine [RDX], and octahydro-13,5,7-tetranitro-1,3,5,7-tetrazocine [HMX] from a series of surfaces, including wet and oily skin). Samples in the subpico-gram level were detected. 

Nelson, B.C., Pfeiffer, C.M., Margolis, S.A., and Nelson, C.P., 2004. Solid-phase extraction-electrospray ionization mass spectrometry for the quantification of folate in human plasma or serum. Analytical Biochemistry. 325 41-51. 

Regarding the sample pretreatment step, traditional extraction techniques that involve the use of high amounts of toxic solvents should be avoided. In this sense, Chingin et al. [95] presented a novel procedure based oti extractive electrospray ionization mass spectrometry (EESI-MS) for the detection of DEP in perfumes with no need of sample pretreatment. Su et al. [96] determined DMP, DBP, dicyclohexyl phthalate (DCHP), and DNOP in perfumes and lacquer removers based on polymer monolith microextraction (PMME) and LC-UV. The conditions for extraction were optimized leading to a sensitive method. 

Chingin K, Gamez G, Chen H, Zhu L, Zenobi R (2008) Rapid classification of perfumes by extractive electrospray ionization mass spectrometry (EESI-MS). Rapid Commun Mass Spectrom 22 2009-2014... 

Desorption electrospray ionization (DESI) [1] was introduced at the end of 2004, and direct analysis in real time (DART) [2] soon after in 2005. The apparent potential of both DESI and DART in high-throughput applications soon led to the development of some derivatives with the intention to broaden the field of applications or to adapt the underlying methodology to specific analytical needs. Now, the repertoire of methods includes variations of the DESI theme such as desorption sonic spray ionization (DeSSI) [3], later renamed easy sonic spray ionization (EASI) [4] or extractive electrospray ionization (EESI) [5,6]. Then, there are the DESI analogs of APCI and APPI, i.e., desorption atmospheric-pressure chemical ionization (DAPCI) [7,8] and desorption atmospheric pressure photoionization (DAPPI) [9]. 

Fig. 13.13. Extractive electrospray ionization interface with two mixing sprays in front of a heated transfer capillary. The distances a and b and the angles a and P are adjusted as required for optimum signal intensity. Adapted from Ref. [37] with permission. The Royal Society of Chemistry, 2006.

EESI Extractive electrospray ionization Sample vapor or mist admixed to electrospray plume [6,37]... 

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